1. Field of the Invention
The present invention relates to feedback control of a drive device such as a motor which stops a control object at a target position.
2. Description of the Related Art
Conventionally, when the position of a control object is to be set, for example, when control is performed such that a control object such as a lever rotated about one point is rotated to a target position, a positioning apparatus having an arrangement shown in FIG. 1 in which the target position is given as a digital value to cause a motor to move the control target to a position near the target position is generally used.
This conventional positioning apparatus comprises an addition circuit 401 for adding a predetermined value to a target value given as a digital value to obtain an upper limit value, a subtraction circuit 402 for subtracting a predetermined value from the target value to obtain a lower limit value, two D/A (digital/analog) converters 403 and 404 for converting the upper and lower limit values into analog values, respectively, a window comparator 405 having outputs from the D/A converters 403 and 404 as two threshold values, and a drive circuit 408 which receives two outputs from the window comparator 405 to perform rotating (angular) control of a motor 409. Since the window comparator 405 receives an output from a PBR (Potentio Balance Resistor) 410 for detecting a present position of the motor 409 serving as a drive Means for moving the position of the control object, the drive circuit 408 checks the present position and the upper and lower limit values on the basis of the two outputs from the window comparator 405 to determine forward rotation, backward rotation or stop of the motor 409. More specifically, if the output from PBR 410 is larger then the upper limit value, the motor 409 is rotated in a direction to decrease the output value of the PBR 410. If the output from PBR 410 is smaller than the lower limit value, the motor 409 is rotated in a direction to increase the output value of the motor 409. In this manner, the output value of the PBR 410 is controlled to be stopped between the upper limit value and the lower limit value.
The interval between the upper limit value and the lower limit value, i.e., a stop range, must be set to be larger than the overrun amount of the control object from when the drive operation of the motor 409 is stopped to when the rotation of the motor 409 is actually stopped so as to prevent hunching.
As described above, in the conventional positioning apparatus, two D/A converters are required to convert the upper and lower limit values into analog values.
In the above arrangement, since the outputs from the two D/A converters independently vary, the interval between the upper limit value and the lower limit value which have been converted into analog values considerably varies in different converters.
FIG. 2 shows the relationship between variations in output from each D/A converter and the interval between an upper limit value and a lower limit value, i.e., a stop range. It is assumed that the value of an output from a D/A converter for converting the upper limit value into an analog amount is smaller than the target upper limit value, that the error is maximum, that the value of an output from a D/A converter for converting the lower limit value into an analog amount is larger than the target lower limit value, and that the error is maximum. In this case, the stop range has a minimum value (range F in FIG. 2). In contrast to this, it is assumed that the value of an output from a D/A converter for converting the upper limit value into an analog amount is larger than the target upper limit value, that the error is maximum that the value of an output from a D/A converter for converting the lower limit value into an analog amount is smaller than the target lower limit value, and that the error is maximum. In this case, the stop range has a maximum value (range G in FIG. 2).
To prevent hunching, the stop range must be larger than the overrun amount of the motor 409 serving as a control object. For this reason, in consideration of variations in D/A converters, addition/subtraction values for a target value must be set such that a set stop range (range E in FIG. 2) is not lower than the overrun amount of the motor 409 serving as a control object even if the stop range has the minimum value (range F in FIG. 2).
On the other hand, the maximum value of the stop range is a factor for determining stop precision for which the control object is stopped at a target position. More specifically, the stop precision which can be assured by a positioning apparatus is the maximum value (range G in FIG. 2) of the stop range determined in consideration of the variation in D/A converter, and this value must satisfy the required precision of a system using this positioning apparatus.
Therefore, when the overrun amount of the control object is close to the stop precision required by the system, prevention of hunching and assurance of the stop precision may not be achieved at once.
In addition, due to the presence of two D/A converters, a circuit scale extraordinarily increases with an increase in the number of bits of a digital value given as a target position. Therefore, problems such as an increase in cost and difficulty in integration are posed.